Microinjection of serotonin directly into the nucleus tractus solitarius produced a dose dependent increase in arterial blood pressure of anesthetized rats. The serotonin antagonists BOL and metergoline significantly attenuated the serotonin pressor effect and the serotonin uptake inhibitor fluoxetine significantly enhanced the magnitude of the pressor response. Electrical stimulation of the dorsal raphe nucleus or microinjections of serotonin into the preoptic region of the hypothalamus produced a transient rise in arterial blood pressure of both spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls. These effects can be attenuated by metergoline. Injections of 5-hydroxytryptophan (5HTP), the immediate precursor of serotonin in brain, lowers blood pressure in the SHR but it appears that this effect is not related to the conversion of 5HTP to serotonin in brain.